The present invention relates to a radio communication system between a radio terminal and a fixed terminal, and/or between radio terminals. In particular, the present invention relates to such a system which provides excellent speech quality avoiding multiple encoding/decoding operations between a radio link and a network.
In order to save bandwidth, and thus reduce transmission costs, in a digital radio communication system, a speech signal is encoded with a low bit rate, for instance 8 kilobits/second (kbps), by exploiting redundancies in the speech signals and compressing the signals. On the other hand, a fixed network uses a coding system with typically 64 kbps.
Therefore, when a radio terminal communicates with a fixed terminal, a coding system must be converted between a low bit rate signal in a radio link and a high bit rate signal in a fixed link.
FIG. 6 shows a prior radio communication system with a network. In the figure, the numeral 1 is a fixed terminal coupled with a network, 2.sub.1 and 2.sub.2 are radio terminals which communicate not only with a fixed terminal, but also with a radio terminal, 3.sub.1 and 3.sub.3 are base stations for coupling radio terminals with the network through radio link, 4 is an exchange station for exchange and control of the terminals 1, 2.sub.1 and 2.sub.2, and 7 is an exchange switch installed in the exchange station 4 for the exchange of the terminals. The numerals 8.sub.1 and 8.sub.2 are codecs for converting a voice code between a radio-specific voice code which is used in a radio terminal, and a wired-line voice code which is used in a network.
When a fixed terminal 1 communicates with a radio terminal 2.sub.1, a codec 8.sub.1 converts a radio-specific voice code of the radio terminal to a wired-line voice code, and vice versa.
Similarly, when the radio terminal 2.sub.1 communicates with another radio terminal 2.sub.2, the codec 8.sub.1 converts the radio-specific voice code from the radio terminal 2.sub.1 to the wired-line voice code which is used in the network, and then, another codec 8.sub.2 converts said wired-line voice code from the codec 8.sub.1 to the radio-specific voice code which is forwarded to the radio terminal 2.sub.2 through the base station 3.sub.2. A voice signal from the radio terminal 2.sub.2 is also converted to the radio specific voice code through the wired-line voice code by two codecs 8.sub.1 and 8.sub.2.
Accordingly, the conventional radio communication system has the disadvantage that when a radio terminal communicates with another radio terminal, a voice code is first converted to a wired-line voice code, and then, converted back to a radio-specific voice code. The process of voice signal twice in codecs deteriorates speech quality in speech distortion and/or delay time.
The U.S. Pat. No. 4,924,480 describes codecs with suppression of multiple encoding/decodings across a connection. In that prior art, a fixed network operates with high bit rate of 64 kbps, and each speech signal is encoded by using 8 bits (B.sub.1 through B.sub.8). A codec communicates its presence to another codec on the high bit rate side is based on the transmission of predetermined synchronization patterns inserted in the signals, for instance the least significant bit B.sub.1. That is to say, one bit of 8 bits of speech encoding is used merely for switching of a codec beween codec-free mode and codec mode, and a speech is encoded by using only 7 bits. As the number of bits for speech encoding is decreased for the switching of a codec, that prior art has the disadvantage that the speech quality is deteriorated.